Reports are increasingly occurring that new calender rollers are becoming “bent” in the calender after a certain running time. These are measured central deviations of the order of 0.1 mm or a bend of about 0.05 mm. The fact that in practice such reports only come from rollers provided with a relatively thin hard coating allows the conclusion that this is not a new phenomenon, but that the incidence of bending has only come to the attention of the calender operator due to the thin coating. For, while it has previously been possible to correct such bending by routinely re-grinding—the re-grindable hard shell has had a thickness of over 8 mm—, correcting by grinding for a layer thickness of 0.1 to 0.15 mm practically always means losing the whole, expensive coating.
When a gray casting body—and the core of clear chill casting rollers such as the invention preferably relates to is such a body—is produced, it experiences practically no mechanical loads during the whole production, beyond the load of the weight of the body. This means that even a slight additional load, e.g. due to the swing of the roller body which is mounted at its ends during transport from the manufacturer to the operator, repeatedly triggers additional tensile stresses which cause a slight, permanent deformation of the roller body. Loads can also for example arise if the quick-release mechanism separates the rollers during operation. This is for example the case if the material web to be treated tears and a heated roller in a calender comes into direct contact with a roller comprising an elastic coating. A specific mechanism then separates the rollers and the rollers fall downwards, where they are collected by a device. This exerts acceleration forces of up to 2 g on the rollers.
Consider the order of magnitude: rollers are produced for example with a diameter of 812 mm and a roller length of 9,000 mm with a production tolerance of 0.005 mm with respect to the concentricity precision. In the case of a hard coating or chromium plating, a layer thickness of 0.15 mm is usually applied. A bend of 0.05 mm means a concentricity error of 0.1 mm, i.e. a deterioration in the tolerance by a factor of 20. At high rotational speeds, this incurs an imbalance and additional excursion.